The present disclosure relates to a current-measuring circuit for redundantly measuring an electric current, having a measuring resistor, an evaluation circuit on an evaluation-circuit printed circuit board and a magnetic-field sensor.
The disclosure also relates to a battery having the current-measuring circuit according to the disclosure and to a motor vehicle having the battery according to the disclosure.
It would appear that, in future, new battery systems will be used both in stationary applications, such as wind turbines, in motor vehicles in the form of hybrid or electric motor vehicles, and in electronic devices, such as laptops or mobile telephones, with very stringent requirements being placed on said battery systems in respect of reliability, safety, performance and service life.
In vehicles with an at least partially electric drive, electrical energy stores are used in order to store the electrical energy for the electric motor which assists the drive or acts as drive. In vehicles of the most recent generation, in this case so-called lithium-ion batteries are used. These batteries are distinguished, inter alia, by high energy densities and an extremely low level of self-discharge. Lithium-ion cells have at least one positive and one negative electrode (cathode and anode, respectively) which can reversibly insert (intercalation) lithium ions (Li+) or extract (deintercalation) them again.
FIG. 1 shows how individual battery cells 10 can be assembled to give battery modules 12 and then batteries 14. This is performed by poles of the battery cells 10 being connected in parallel or series (not illustrated). In this case, by definition, a battery module 12 or a battery 14 comprises at least two battery cells 10, wherein the terms battery 14 and battery module 12 are often used synonymously. The electric voltage of a battery 14 is, for example, between 120 and 600 volts DC.
In the case of batteries for automobile drive technology (traction batteries), there is a need, to determine the state of charge and for reasons of safety, to measure the current supplied to and discharged from the battery cells. Therefore, the functionality of the current sensor must be known and hence be detectable by means of suitable measures. In many cases, the current is detected using current sensors which operate on the basis of the resistor principle (shunt). In order to keep the power loss in the shunt resistor to a minimum, the resistance value thereof is selected to be much smaller than that of the load. The small voltage drops arising therefrom must therefore be amplified for evaluation by means of electronic circuits connected downstream and evaluated. If the connection between resistor and evaluation circuit is interrupted, or if the resistor or the circuit has another fault, this cannot be readily unambiguously ascertained.
DE 10 2009 046 564 A1 discloses a battery system having a high-voltage system and a low-voltage system. The high-voltage system comprises a battery module, while the low-voltage system comprises a battery control unit (BCU). Cell-monitoring units are assigned to the battery cells and measure the voltages of the battery cells. Furthermore, the document discloses a redundant current measurement by means of a measuring resistor (shunt) and a Hall sensor.
The disclosure provides a current-measuring circuit for redundantly measuring an electric current, having a measuring resistor, a magnetic-field sensor and an evaluation circuit on an evaluation-circuit printed circuit board. The evaluation circuit is designed to determine the electric current using the measuring resistor. The magnetic-field sensor on the evaluation-circuit printed circuit board and the evaluation-circuit printed circuit board are characteristically arranged in the immediate vicinity of the measuring resistor, with the result that the magnetic-field sensor can detect the magnetic field of the measuring resistor when current flows through it.
The disclosure is based on the discovery that, by integration of the magnetic-field sensor on the evaluation-circuit printed circuit board, for example in the context of a placement option, a particularly compact and inexpensive redundant current-measuring circuit in one unit is made possible, which current-measuring circuit is based on two different measurement principles. This is made possible by virtue of the fact that the magnetic-field sensor detects that magnetic field which builds up around the measuring resistor owing to the flow of current in the measuring resistor.
When selecting the measuring resistor, it is advisable to use a measuring resistor—also called shunt—which is as small as possible. Preferably, measuring resistors with an electrical resistance of less than 1 mΩ are possible, in order to keep the losses caused by the measuring resistor to a minimum. The use of a small measuring resistor means that even only small voltage drops can be measured across said measuring resistor during operation though. Said small voltage drops are, as usual, amplified and evaluated using the evaluation circuit and are used as a measure for the current flowing through the measuring resistor. Owing to the relatively high accuracy of such circuits, the signal obtained therefrom is used as primary signal for precise determination of the current flowing through the measuring resistor.
In addition to the evaluation circuit, the magnetic-field sensor is arranged on the evaluation-circuit printed circuit board and is designed to measure a magnetic field around the measuring resistor, which magnetic field is proportional to the flow of current; the magnetic-field sensor is preferably embodied as a Hall sensor. The secondary signal output by the magnetic-field sensor in turn corresponds to the current flowing through the measuring resistor, which current is thus redundantly calculated on the basis of a second independent measurement principle.
According to an advantageous configuration of the disclosure, the evaluation circuit is also designed to compare the primary signal corresponding to the current determined by means of the measuring resistor with the secondary signal corresponding to the current determined by means of the magnetic-field sensor and to output an error signal in the event that the two signals differ. This can be achieved by integrating a few additional components, for example, evaluation components, on the evaluation-circuit printed circuit board. The primary signal, which is determined via the measuring resistor and the evaluation circuit, generally allows more precise conclusions to be drawn in relation to the flow of current than the secondary signal of the magnetic-field sensor. However, should the primary signal suddenly differ significantly from the secondary signal of the magnetic-field sensor, the error signal is output, as a result of which a fault in the current-measuring circuit can be identified.
So that the magnetic-field sensor can be operated in an optimum manner, said magnetic-field sensor is arranged in the immediate vicinity of the measuring resistor. The normal distance between the magnetic-field sensor and the measuring resistor is preferably less than 10 mm, in particular less than or equal to (≤) 5 mm. The smaller the normal distance, the better the interference immunity with respect to external interfering magnetic fields.
It is further preferred that the evaluation-circuit printed circuit board having the magnetic-field sensor lies flat against the measuring resistor. In this case, it may be necessary to electrically isolate the evaluation-circuit printed circuit board from the measuring resistor, apart from two electrically conductive connections thereto. The voltage drop across the measuring resistor can be measured by means of the two electrically conductive connections to the measuring resistor.
Furthermore, a battery having a current-measuring circuit is provided. The current-measuring circuit is used, for example, to measure currents within the battery.
The battery is preferably a lithium-ion battery. Owing to the use of lithium-ion technology by means of a plurality of lithium-ion secondary cells, particularly high energy store densities can be achieved, which leads to further advantages, in particular in the field of electromobility.
Furthermore, a motor vehicle having the battery according to the disclosure is provided. The battery is generally used to supply energy to the drive of the motor vehicle. The battery according to the disclosure can also advantageously be used in an electrical device.
Advantageous developments of the disclosure are specified in the dependent claims and can be gleaned from the description.